SAMM 368, a transplantable myeloma of BALB/c mice was established in long term tissue culture. The myeloma tumor produces two paraproteins in vivo, and IGA and an IgG2b. Five established culture lines were examined. Three produced both paraproteins, one produced only the IgA, and one produced only the IgG2b. Thirty-five clones of one of the double producers were all double producers.